1. Field of the Invention
This invention relates in general to a bearing structure used in a pivot bearing for freely supporting the rotation of a head arm of a hard disk drive (HDD), and more particularly to a bearing structure which eliminates the temperature dependence of the rotational torque of a bearing.
2. Description of Related Art
In a hard disk drive, a magnetic disk is installed on a spindle motor and driven to rotate, and an actuator assembly is provided. According to the actuator assembly, an arm having a magnetic head at one end thereof is rotated with an actuator using a voice coil motor (VCM) to position the magnetic head over the magnetic disk so that reading and writing of magnetic data can be performed.
A pivot bearing is employed in the actuator assembly to freely support the rotation of the head arm. In the pivot bearing, a pair of bearings is spaced in the axial direction. In each bearing, the outer race is permanently attached to the outer sleeve of the head arm side, and an inner race is integrally formed with the inner shaft extending vertically from a base. A preload is exerted on the pair of bearings.
The hard disk drive is, incidentally, required to provide high-speed operation, be compact and inexpensive, and to deliver a low power dissipation.
For this reason, it is particularly necessary in the pivot bearing to make the startup torque small, because the startup and stopping of the pivot bearing are repeated. The startup torque is determined by the preload to be exerted on the pivot bearing and the nature or quantity of lubricant to be enclosed in the pivot bearing. If the preload is high, the startup torque will become large and, if the quantity of the lubricant is increased, the startup torque will become large.
With regard to the preload, if the coefficients of linear expansion of the inner shaft and the outer sleeve are different, the preload will be increased by the rise in temperature and the preload will be decreased by the fall in temperature, which means that the startup torque changes. In the conventional pivot bearing, the quality of the material of the inner shaft and the quality of the material of the outer sleeve are made the same to avoid having the preload become temperature-dependent.
However, on the one hand, if the bearing steel is used in both the material of the inner shaft and the material of the outer sleeve, passivation processing will be required to attach the outer sleeve to the outer race and it will also be necessary to use a coating of primer (hardening accelerator) prior to attachment. This increases the cost of the bearing.
On the other hand, when lubricant is enclosed in the pivot bearing, the startup torque becomes smaller as the lubricant temperature rises and larger as the lubricant temperature falls, because of the characteristic of the lubricant, as shown by the graph 400 in FIG. 4. FIG. 4 illustrates four lubricants 402, 404, 406, 408 with different coefficients of expansion. It is to be noted that, in the graph in FIG. 4, the axis of the ordinates represents temperature and the axis of the abscissas represents rotational torque, and each curve shows the characteristic of the lubricant at 1800 rpm for the various types of lubricants 402, 404, 406, 408.
Thus, it has been the practice to reduce the influence of the characteristic of the lubricant to a minimum by severely controlling the quality of material and the quantity of the lubricant, but this has its limitations and, consequently, the temperature dependence of the startup torque by the lubricant still remains.
It can be seen then that there is a need for a bearing structure which eliminates the temperature dependence of the rotational torque of a bearing and makes possible high-speed operation, low power dissipation, and other features that are required of a hard disk drive and in which the deterioration in the performance of the hard disk drive does not occur even under a temperature change.